Data communications often involves the use of modulator/demodulators (modems) or modem-like devices (e.g. Digital Subscriber Line devices, 56 kbps digital "modems", etc.) to communicate over a network system having at least one network link that uses an analog signaling scheme. For example, as shown in FIG. 1, two computers 1, 2 may each have a modem link 3, 4 (land-line or wireless) to the public switched telephone network 5. Generally, at least part of the virtual circuit connecting the two computers 1, 2 will be analog in nature.
A characteristic of analog links is that various components in the link, such as 2-wire to 4-wire hybrids, can cause echos in the link. Further, variations in analog component values can change operating characteristics of the link. Accordingly, under many current standards, such as the ITU V.34 standard defining a 33,600 kbps data signaling rate, modems conforming to the standard perform various training routines to set equalization and echo canceling parameters before commencing data transmission. For example, FIG. 2 shows a bi-directional signaling diagram of one type of routine, in which modem A is conducting identification and line probings routines with a second modem B (not shown). Initially, modem A transmits a known training sequence, PSA, to modem B for a preset, minimum time period (for example, about 0.15 secs.). Using the PSA sequence, modem A trains its echo canceling circuitry while modem B trains its equalizer circuitry, in known fashion. After the PSA sequence is transmitted for no more than a preset, maximum time period, modem A transmits an end-of-sequenice marker, EOSA.
After receiving the EOSA marker, modem B transmits a known training sequence, PSB, to modem A for a preset, minimum time period (for example, about 0.15 secs.). Using the PSB sequence, modem B trains its echo canceling circuitry while modem A trains its equalizer circuitry, in known fashion. After the PSB sequence is transmitted for no more than a preset, maximum time period, modem B transmits an end-of-sequence marker, EOSB.
Under the V.34 standard, this initial round of training is at a relatively low rate, typically about 6,000 bps. A second round of training at a low-to-mid rate (e.g., about 12,000 bps) then takes place, in similar fashion. That is, after receiving the EOSB marker, modem A transmits a known training sequence, PSA', to modem B for a preset, minimum time period (for example, about 0.15 secs.). Using the PSA' sequence, modem A trains its echo canceling circuitry for a higher data rate while modem B trains its equalizer circuitry, in known fashion. After the PSA' sequence is transmitted for no more than a preset, maximum time period, modem A transmits an end-of-sequence marker, EOSA'. Modem A is then fully trained, and may begin to transmit user data, DataA, at the full connected modem rate. Meanwhile, modem B performs this second round of training concurrently with training for modem A. That is, essentially immediately alter the EOSB marker, modem B transmits a known training sequence, PSB', to modem A for a preset, minimum time period (for example, about 0.15 secs.). Using the PSB' sequence, modern B trains its echo canceling circuitry for a higher data rate while modem A trains its equalizer circuitry, in known fashion. After the PSB' sequence is transmitted for no more than a preset, maximum time period, modem B transmits an end-of-sequence marker, EOSB'. Modem B is then fully trained, and may begin to transmit user data. DataB, at the full connected modem rate.
Such standard training may take from 5 to 15 seconds for high-speed (e.g., 14,400 bps or faster) modems before user data can be transmitted or received by either modem. Such a delay is undesirable for "transaction modems", which are typically used at point-of-sale (POS) terminals to conduct such transactions as check verification, credit card validation, etc. Slower modems (e.g., 300-1,200 bps) having faster training times have often been used as transaction modems, and can be satisfactory if transactions always involve a small amount of data (e.g., ten's of bytes of data, sufficient, for example, for validating a credit card number). However, such modems are undesirable if a substantial amount of data must be transmitted more than occasionally. Thus, more and more frequently, high-speed modems are being used for transaction applications. Similarly, such training times for standard modems adversely impacts the response time of modem access to the Internet.
One way of shortening the training time of high-speed modems is to only train for the minimum time allowed by a data signaling standard. However, this approach results in a lower data rate at the end of the training period than if training had been performed for a greater time period.
Thus, the inventors have determined that it would be desirable to make high-speed modems more efficient, particularly for transaction and Internet applications.